U.S. 2007/0171679 discloses a series-parallel resonant converter or zero-voltage-switching converter with a charge mode control. This converter has two different modes, a frequency mode and a charge mode. In the frequency mode, a load circuit signal is fed back. In the charge mode, a transformer circuit signal is fed back. For small loads, the converter is in the frequency mode. For high loads, the converter is in the charge mode.
In general, resonant converters are relatively sensitive to feeding voltage fluctuations and to load variations.